File paths to system partition are okay-ish.

Unless SELinux blocks it, all apps have identical access to files
included on the system partition.  Since there are a handful of
useful files stored there, like ringtones and license files, carve
out an exception to allow file:///system/ style paths.

Note that StrictMode isn't a security mechanism, which is why we're
not concerned about resolving canonical paths.

Bug: 26895798
Change-Id: If0b659d30c4e51377edcf01445392759d1e4962e